1. Field of the Invention
The present invention relates to a data service providing method in a communications system, and in particular, to a method for providing data services at high speed via complemental channels.
2. Description of the Related Art
The highest bit rate per user in a CDMA (Code Division Multiple Access) system is 9.6 kbps/14.4 kbps. Many methods have been suggested to exceed this data rate limit and provide data services at high speed. One of them, a multi-code scheme, assigns a plurality of (eight in maximum) code channels to one user and offers him data services at a maximum bit rate of 14.4 kbps.times.8.
Despite its ability of implementing various supplemental functions for the user, the method of providing high-speed data service has a drawback in that as the user can transmit and receive a larger amount of data through the high-speed data service, a CDMA mobile communications network becomes accessible to less users. This implies that an increase or decrease in the number of users may bring a rapid change to the bit rate (number of users .times.bit rate per user) of the entire communications network. In this case, the stability of the CDMA mobile communications network may be impaired.
Accordingly, this problem should be minimized and reliable provisions of existing basic services should be ensured, for high-speed data services.
FIG. 1 is a block diagram of a terminal, a base station, and a radio link in a mobile communications system. The radio link of FIG. 1 is composed of a forward channel transmitted from the base station to the terminal and a reverse channel transmitted from the terminal to the base station.
A conventional CDMA mobile communications system following the IS-95/IS-95A/J-STD-008 standard has a forward channel structure as shown in FIG. 2A and a reverse channel structure as shown in FIG. 2B. The forward CDMA channel has a pilot channel, a sync channel, a paging channel, and a forward traffic channel, as shown in FIG. 2A. The reverse CDMA channel includes an access channel and a reverse traffic channel, as shown in FIG. 2B.
The forward code channels contain 64 orthogonal Walsh codes, with one forward traffic channel occupying one Walsh code. Thus, the full transmission bit rate of each forward or reverse traffic channel is 9.6 kbps/14.4 kbps. To provide data service at high speeds exceeding 14.4 kbps in the above CDMA channel structures, at least one forward Walsh code and one reverse Walsh code should be allowed to a user.
For this purpose, the forward and reverse CDMA traffic channels of FIGS. 2A and 2B should be further divided as shown in FIGS. 3A and 3B, respectively. In a new CDMA channel structure, the forward channel includes a pilot channel, a sync channel, a paging channel, and a forward traffic channel divided into a fundamental channel and a supplemental channel, while the reverse channel includes an access channel and a reverse traffic channel divided into a fundamental channel and a supplemental channel.
The high-speed data service in the above channel structures refers to a multi-code scheme. The fundamental channels of FIGS. 3A and 3B support the same functions and bit rates as those of the conventional forward and reverse traffic channels. The supplemental channels are assigned to a user upon his request for high-speed data service, and are not used in the absence of a user request. 0 to 7 supplemental channels are available to the user depending on their request.
When a user is to receive high-speed data service via the supplemental channels, connection information should be first obtained from a corresponding base station via the pilot channel, sync channel, and paging channel. Then, a user terminal attempts to connect with the base station via the access channel, using the obtained information. With a successful connection via the access channel, a connection path is formed between the user terminal and the base station. Since then, the connection between them is maintained via the fundamental channels.
Generally, data services that the user desires have a burst-type quality depending on the amount of data transmitted. That is, data to be transmitted or received instantaneously does not always exist, and a data transmission period is coexistent with a data non-transmission period. In this context, only when a call is set up between the terminal and the base station via the fundamental channels, and it is necessary to transmit data at high speed from a user or the base station, the request for high-speed data transmission is notified to the other party, and a high-speed data service is provided via the supplemental channels. Via the fundamental channels, the negotiation is performed, and completion of the data transmission is notified. Then, the supplemental channels stop their action.
The above procedure is exemplarily illustrated in FIG. 4, on the part of the terminal.
In step 411, a call is set up between the terminal and the base station via the fundamental channels. In step 413, the terminal determines whether high-speed data transmission data exists. If so, the terminal sends a request for use of a supplemental channel to the base station via a reverse fundamental channel (i.e., the base station commands the terminal to use a supplemental channel via a forward fundamental channel), in step 415. In step 417, the base station determines whether the requested supplemental channel can be designated (the terminal determines whether the commanded supplemental channel can be received). If the designation is impossible, the base station notifies the terminal of the unavailability of the supplemental channel via the forward fundamental channel (if the reception is impossible, the terminal notifies the base station of the reception impossible via the reverse fundamental channel), and the call is maintained via the forward and reverse fundamental channels, in step 419. Then, the procedure goes back to step 413.
On the contrary, if the supplemental channel is available in step 417, the base station gets the supplemental channel ready, and notifies the terminal of the readiness of the supplemental channel via the forward fundamental channel (and the terminal notifies the base station that the terminal is ready via the reverse fundamental channel), in step 421. In step 423, data is transmitted between the terminal and base station at high speed via the fundamental and supplemental channels. In step 425, it is determined whether the high-speed data transmission/reception is completed. If it is not completed, steps 423 and 425 are repeated.
If the high-speed data transmission/reception is completed in step 425, the base station and the terminal notify the other parties of completion of the transmission/reception via the forward and reverse fundamental channels, the supplemental channels stop their action, and the call is maintained via the fundamental channels, in step 427. In step 429, it is determined whether the call is completed, while maintaining the call. If the call is not completed, the procedure goes back to step 413. Otherwise, a call completion process is performed, in step 431.
As shown in FIG. 4, the multi-code using high-speed data service exhibits temporal code channel occupation characteristics due to data transmission and reception in the form of packets. That is, code channels are concentratedly busy for a predetermined time, but they are idle for other time periods.
FIG. 5 illustrates use of forward code channels when high-speed data service is provided to two users via supplemental channels. The horizontal axis indicates time, and the vertical axis indicates the number of code channels used, which eventually represents the forward CDMA channel load. That is, the load increases with the number of the code channels being used.
In FIG. 5, F represents a fundamental channel, and a number attached to F represents a user number. Thus, F1 indicates a fundamental channel for user 1. S indicates a supplemental channel, and first and second numbers attached to S indicate a user number and the number of a supplemental channel used, respectively. Thus, S2,3 indicates supplemental channel 3 for user 2.
However, the conventional high-speed data service using a multicode as shown in FIG. 4 raises the problem that the total forward bit rate is not constant and rapidly changes when high-speed data service is provided on the forward supplemental channel.
The changing of the total bit rate prevents full use of an available CDMA channel capacity, thereby wasting channel capacity. Assume that every data is transmitted at a constant rate from the base station, table 1 is listed from a received packet M/M1/queuing model.
TABLE 1 waiting time of packet wasted capacity twice transmission time 33% ten times transmission time 9%
Table 1 is obtained by ##EQU1## .rho..sub.0 =1-.rho. (1)
where W is a waiting time, 1/.mu. is an average transmission time, .rho. is .lambda./.mu., .lambda. is an input rate, .rho. is a use rate, and .rho..sub.0 is a non-use rate. In the above case, supplemental channels are used as shown in FIG. 5. Idle time slots are generated when data service is provided within a predetermined capacity as shown in FIG. 5, resulting in a waste of capacity.
Another problem, namely, the rapid change of bit rate indicates a rapid change in the forward load, which is likely to impede stable cell operations. Assume that the ratio of pilot power to voice user power to data user power is 2:0.4:8 where 0.4 is a voice activity and 8 indicates use of eight channels used at a full rate. Also assuming that there are 10 users in each of, for example, two cells, data users are confined to one cell, and a base station uses a given total power, the radius of the data user cell is reduced by 10% in effect. If the base station outputs a constant pilot power, Ec/Io (a key parameter of handoff) reported from the terminal can vary by 2 dB depending on the presence or absence of a data user.
The above-described two problems impede stable communications network operations and cause channel capacity consumption.